The invention relates to a dielectric filter with an input an output for a microwave signal and a rotationally symmetrical dielectric resonator body that can be induced by the microwave signal to execute electromagnetic oscillations. A filter of this kind is described, for example in DE 196 176 98 C1.
The invention also relates to a method for producing dielectric filters as well as a method for adjusting the mode coupling in dielectric filters.
Filters for high-frequency signals, in particular microwave signals, are used in large numbers in satellites. Due to the very high transport costs of satellite payloads, designers strive to achieve filter functions predetermined by the purpose of the satellite by using filters that are as light in weight and low in volume as possible. Because of the very high qualities required, cavity resonator filters are often used. As described in the reference xe2x80x9cApplication of Dual TM Mode to Triple and Quadruple-Mode Filtersxe2x80x9d, Renxc3xa9 R. Bonetti and Albert E. Williams, IEEE Transactions on Microwave Theory and Techniques, vol. MTT-35, No. 12, December 1987, pp. 1143 to 1149, one method for better exploiting the volume is to use dual-mode-, triple-mode-, or quadruple-mode filters. Due to a symmetry of their geometric form, these filters have degenerated modes, one of which is induced via the signal input of the filter. A slight deviation of the filter form from the absolute symmetry achieves a coupling of the induced mode into a degenerated, orthogonal mode. The thus induced mode canxe2x80x94in the case of a dual-mode filterxe2x80x94be read at its output as an output signal orxe2x80x94in the case of higher multiple-mode filtersxe2x80x94can be used to induce another degenerated mode. The action of this kind of multiple-mode filter corresponds to a series connection of a number of mono-mode filters in a bridge part of their volume and weight.
Another possibility for reducing the space requirement of filters is to use dielectric work pieces. By using these, the linear dimensions of the filter can be reduced proportional to the square root of the relative permittivity. An example of a filter in which the two methods are combined is disclosed in U.S. Pat. No. 4,489,293 A. However, in comparison to a cavity resonator, reducing the structure produces greater wall currents in the metallic periphery of the filter housing surrounding the dielectric resonator and therefore results in a reduction in resonator quality. Therefore, a compromise is generally required. The limiting metal surfaces are affixed a certain distance from the dielectric.
In order to be able to reduce this distance without having to accept a reduction in resonator quality, it is necessary to find resonator geometries in which the fields of the technically relevant resonator modes emitted by the dielectric resonator body are relatively minor so that they interact only slightly with the surrounding metal surfaces. The reference DE 196 176 98 C1 cited above suggests using a hemisphere as a resonator body. This hemisphere is affixed with its flat surface on a high temperature super-conducting plate. In a variant known from the reference xe2x80x9cHigh Temperature Super-Conductor Shielded High Power Dielectric Dual-mode Filter for Applications in Satellite Communicationsxe2x80x9d, S. Schornstein, I. S. Ghosh, and N. Klein, IEEE MTT-S Digest, pp. 1319 to 1322, 1998, a hemispherical resonator body is also used, which is spaced apart from a metallic shielding surface by a pedestal made of dielectric material.
Because of its low dielectric losses, preferably a monocrystalline lanthanum aluminate or the like is used as a material for the resonator body. However, producing a hemispherical dielectric body of this material is not easy for a number of reasons. Since the material is very hard and brittle, the form can only be produced through grinding. In order to grind a precisely curved surface, a high precision, numerically controlled grinding machine must be used. This type of manufacture is very time-consuming and very expensive. The resonance frequency of the resonator body is affected by its form and depends on the relative permittivity of its material. A fine-tuning of this resonance frequency in a hemispherical resonator body is only possible within narrow limits.
Since the relative permittivity of the raw material is subject to fluctuations, a test specimen must of each raw material shipment must be prepared and then the precise geometry of the resonator body to be produced must be defined if a predetermined resonance frequency is to be produced.
The current invention produces a dielectric microwave filter which can be inexpensively manufactured and can be simply tuned to a required resonance frequency. These advantages are attained in a dielectric filter of the type mentioned at the beginning with the aid of a resonator body which has two different-sized basal surfaces perpendicular to its rotational symmetry axis and has side surfaces which connect the basal surfaces along straight lines. A resonator body of this kind can be produced rapidly and inexpensively by means of a simple circular grinding and/or face grinding.
The proportions of the basal- and side surfaces are suitably chosen so that the resonator body resembles a hemisphere in order to achieve a mode structure of the natural oscillations of the resonator body which resembles that of a hemisphere and has correspondingly few field components outside the resonator body.
In a simple embodiment, the resonator body can be in the form of a truncated cone or a truncated pyramid with a number of sides that is in principle arbitrary.
On one of its basal surfaces, preferably the large basal surface, the resonator body preferably has a pedestal which serves to fasten the resonator body in a housing, with a spacing between a metallic housing wall and the basal surface that has the pedestal.
Preferably, the filter according to the invention is a multiple-mode filter. A screw, which is fastened in the housing of the filter and engages in an inner chamber of the filter encompassing the resonator body, can serve in the usual manner as a symmetry interrupting element or mode coupler. However, a symmetry interruption can also be produced by virtue of the fact that one of the basal surfaces of the resonator body, at least in part, extends at a slight inclination in relation to the other basal surface.
In the effective frequency band of a dielectric filter, there can also be higher oscillation modes whose fields are concentrated in the vicinity of the surface of the resonator body. Modes of this kind are heavily influenced by the surroundings of the resonator body, in particular by the housing, and are thus poorly suited to filtering applications. In order to suppress such modes or to shift them out of the effective frequency band, dielectric material can be locally applied to and/or removed from the resonator body. Such local changes have only a slight influence on modes whose fields are concentrated on the interior of the resonator body.
A dielectric body with at least one flat basal surface, such as the resonator body of the filter according to the invention, is well-suited in terms of the fine-tuning of its resonance frequency through the removal of material from the basal surface. It is therefore possible to mass produce resonator bodies of this kind as blanks; in these blanks, dispersions of the resonance frequency, for example due to differences in the relative permittivity of the raw material, can be taken into account and each blank can then be fine-tuned to a desired resonance frequency through the removal of material from the basal surface.
Other features and advantages of the invention ensue from the following description of exemplary embodiments.